In recent years, with the development of portable devices such as personal computers and mobile phones, there has been rapidly increasing demand for batteries usable as a power source thereof. In cells of the batteries for the purposes, a liquid electrolyte (an electrolytic solution) containing a combustible organic diluent solvent has been used as an ion transfer medium. The cell using such an electrolytic solution can cause problems of solution leakage, ignition, explosion, etc.
In view of solving the problems, all-solid-state cells, which use a solid electrolyte instead of the liquid electrolyte and contain only solid components to ensure intrinsic safety, have been developed. The all-solid-state cell contains a sintered ceramic as the solid electrolyte, and thereby does not cause the problems of ignition and liquid leakage, and is hardly deteriorated in battery performance by corrosion. Particularly all-solid-state lithium secondary cells can achieve a high energy density easily, and thus have been actively studied in various fields.
In general, the all-solid-state cell is experimentally produced by applying a cell active material to a plane surface of the solid electrolyte and by firing the resultant to form an electrode. In this production, the connection area between the solid electrolyte and the cell active material does not exceed the plane area of the cell. The connection area is practically the total of contact areas between particles of the solid electrolyte and the cell active material, and thereby is generally smaller than the surface area of the electrode, resulting in a high resistance against charge transfer between the solid electrolyte and the cell active material.
In view of increasing the contact area between the solid electrolyte and the cell active material, thereby lowering the charge transfer resistance therebetween, for example, an all-solid-state secondary lithium cell having a composite electrode formed by filling pores of a porous solid electrolyte with a cell active material is disclosed in Japanese Laid-Open Patent Publication No. 2006-260887, etc.
More specifically, the all-solid-state secondary lithium cell is obtained by filling pores of a porous solid electrolyte having a lithium ion conductivity of 0.5×10−4 S/cm or more with a cell active material capable of acting as a positive or negative electrode material in the cell, thereby forming a secondary lithium cell electrode composed of a composite of the porous solid electrolyte and the cell active material, and by connecting an electrolyte and a counter electrode thereto.
Such a cell has a contact interface area between the solid electrolyte and the cell active material equal to the surface area of the porous body, and thereby has a charge transfer resistance lower than that of the cell produced by applying the cell active material to a plane surface of the solid electrolyte.
However, the secondary lithium ion cell described in Japanese Laid-Open Patent Publication No. 2006-260887 has the following disadvantages since the further electrolyte has to be connected to the composite electrode composed of the porous solid electrolyte and the cell active material.
(a) When the composite electrode and the electrolyte are insufficiently connected, the connection interface may have a high resistance.
(b) The production of the cell requires the additional process of connecting the composite electrode and the electrolyte.
To form the connection interface satisfactorily, it is necessary to integrate the composite electrode and a dense body of the electrolyte by continuously applying a high pressure or by another process such as sintering. However, the porous composite electrode has a brittle structure, and has the following disadvantages.
(c) The composite electrode cannot be easily handled in the process of pressurization or sintering.
(d) The composite electrode is often cracked in the process of pressurization or sintering.
Particularly, when the composite electrode and the electrolyte have small thicknesses for lowered cell resistance, the problems of (c) and (d) are quite often caused in the integration.